Fix recently introduced use of nullptr in wxGenericListCtrl

Correct the too hastily backported change of cb69e76 (Fix not
refreshing generic wxListCtrl after deleting last item, 2023-10-06)
and use NULL instead of nullptr in this branch.

Author: vadz

src/generic/listctrl.cpp

@@ -1998,7 +1998,7 @@ void wxListMainWindow::RefreshAfter( size_t lineFrom )
         // even if lineFrom is invalid because it may have been (just) deleted
         // when we're called from DeleteItem().
         size_t visibleFrom;
-        GetVisibleLinesRange(&visibleFrom, nullptr);
+        GetVisibleLinesRange(&visibleFrom, NULL);
 
         if ( lineFrom < visibleFrom )
             lineFrom = visibleFrom;